The Biology of What Is Not There

Understanding the living world seems challenge enough for biologists. Technological advances have produced a cascade of data—from detailed genome sequences to the sophisticated satellite imagery that documents the planet’s ecosystems—but our ability to make sense of these data still lags far behind their acquisition. With such a backlog, the idea that we might want to think about objects that are not real seems, frankly, capricious.

Yet without thinking about life-forms that never were, we cannot fully understand the life forms that do exist. Questions about absence lie at the very core of evolutionary biology: Is the living world we behold the only possible outcome of the evolutionary forces that produced it? Or can we imagine, instead, different unfoldings, of which this is but one? The one actual unfurling of the tree of life, of course, places restrictions on our capacity to address this question. I would argue that there are nonetheless ways of posing such queries that may prove productive.